The dosage of treating chemicals in the bleaching or delignification of cellulosic pulp is difficult to control accurately because the flow rate of pulp in most continuous pulp treating processes varies with time, and the direct measurement of pulp flow rate is difficult because the pulp contains two or three phases (fibers, liquor, and optionally dispersed gas) and often is not completely homogeneous. Known methods of pulp flow measurement are often applied at a location in a pulp treating sequence removed from the location of chemical addition, and because of lag times and pulp inventories a pulp flow rate measured at one location is not representative of the flow rate at another location in the process. Inaccurate dosage of chemicals is undesirable for several reasons. Overdosage wastes treating chemicals and may affect downstream process steps adversely, and also may result in poor pulp properties; underdosage results in incompletely treated pulp; and cyclic underdosing and overdosing yields a nonhomogeneous pulp product.
In oxygen delignification and bleaching processes, the flow of oxygen to the pulp-oxygen mixing device prior to the reactor is generally set in excess of the actual requirement and controlled according to a pulp flow signal and pulp consistency measurement. Pulp properties such as Kappa number, brightness, and viscosity are determined periodically by laboratory analyses, and these results may be used judgmentally to adjust the oxygen dosage. Online Kappa number analyzers are available and have found some use for feedback or feedforward control of oxygen dosage. In the feedback mode, online Kappa number measurement can be used for oxygen flow control independent of pulp flow rate. However, a sampling/analysis time of at least five minutes is required for this type of analyzer, and this lag time can adversely affect control performance. In addition, the Kappa number is measured on treated pulp discharged from a reactor which may have a residence time of up to one hour, which also increases control lag time. When online Kappa number measurement is used in the feedforward mode, pulp flow rate measurement is required, and the difficulties associated with pulp flow rate measurement adversely affect this control mode. Online Kappa number analyzers are expensive and maintenance-intensive.
The measurement of residual oxygen gas in the reactor offgas is an alternative method of determining and controlling oxygen dosage. In this method, the flow rate of the offgas is irregular and difficult to measure; instead, the residual oxygen concentration is measured, and the amount of excess oxygen is determined indirectly by knowing the quantity of air entrained in the pulp or by adding an inert tracer gas such as helium to the oxygen before dosing the pulp. This determination of excess oxygen is then used to adjust the oxygen flow rate to maintain oxygen dosage at the desired level. This method is useful for approximate correction of oxygen dosage at specific time intervals, but is not suited for continuous online control of oxygen dosage because the excess oxygen is determined at the reactor outlet, which introduces a large lag time into the control system as earlier discussed.
None of these previous methods for controlling chemical dosage, particularly oxygen dosage, to cellulosic pulp is completely satisfactory. There is no known online method for determining the amount of oxygen in a pulp immediately after oxygen addition prior to the reactor, which would eliminate the lag time associated with determining excess oxygen in the reactor discharge. In addition, methods for accurate, direct measurement of pulp flow rate do not exist. The method of the present invention, as defined in the following specification and claims, allows the online control of chemical dosage, particularly oxygen dosage, to a pulp without the need for direct pulp flow rate measurement.